A Universal Spinning‐Coordinating Strategy to Construct Continuous Metal–Nitrogen–Carbon Heterointerface with Boosted Lithium Polysulfides Immobilization for 3D‐Printed Li—S Batteries

Abstract Constructing intimate coupling between transition metal and carbon nanomaterials is an effective means to achieve strong immobilization of lithium polysulfides (LiPSs) in the applications of lithium–sulfur (Li—S) batteries. Herein, a universal spinning‐coordinating strategy of constructing continuous metal–nitrogen–carbon (M—N—C, M = Co, Fe, Ni) heterointerface is reported to covalently bond metal nanoparticles with nitrogen‐doped porous carbon fibers (denoted as M/M—N@NPCF). Guided by theoretical simulations, the Co/Co—N@NPCF hybrid is synthesized as a proof of concept and used as an efficient sulfur host material. The polarized Co—N—C bridging bonds can induce rapid electron transfer from Co nanoparticles to the NPCF skeleton, promoting the chemical anchoring of LiPSs to improve sulfur utilization. Hence, the as‐assembled Li—S battery presents a remarkable capacity of 781 mAh g−1 at 2.0 C and a prominent cycling lifespan with a low decay rate of only 0.032% per cycle. Additionally, a well‐designed Co/Co—N@NPCF‐S electrode with a high sulfur loading of 7.1 mg cm−2 is further achieved by 3D printing technique, which demonstrates an excellent areal capacity of 6.4 mAh cm−2 at 0.2 C under a lean‐electrolyte condition. The acquired insights into strongly coupled continuous heterointerface in this work pave the way for rational designs of host materials in Li—S systems.

min -1 to make the PS template completely decomposed. The coordinated PS@PDA-Metal n+ organic/inorganic precursor was simultaneously converted to a strongly coupled hybrid material comprised of nitrogen-doped porous carbon nanofiber embedded with cobalt nanoparticles, which was signed as M/M-N@NPCF.
In addition, an aqueous solution of dopamine (2 mg mL -1 in 10 mM Tris buffer, pH = 8.5) and Co(Ac) 2 ·4H 2 O (0.1 M) without the addition of porous PS fiber template was prepared and mildly stirred for 24 h to collect PDA@Co 2+ powder. Then, the PDA@Co 2+ powder was carbonized at the same condition to obtain a comparison sample of nitrogen-doped carbon embedded with cobalt nanoparticles (Co/Co-N@NC).
Based on a melt-diffusion method, the mixed powder, consisting of the host material of M/M-N@NPCF hybrid and sulfur at a weight ratio of 1:2, was transferred to a sealed vessel after grinding for several minutes, and heated at 155 °C for 12 h to fabricate the M/M-N@NPCF-S composite. For comparison, the sulfur loaded nitrogen-doped porous carbon fiber (NPCF-S) was prepared by the same method except for immersing metal salt solution.

Preparation of plain M/M-N@NPCF-S cathode
Typically, the electrode materials of M/M-N@NPCF-S and NPCF-S were respectively mixed with carbon black and poly(vinyl difluoride) (PVDF) at a weight ratio of 8:1:1. The slurry was then cast onto aluminum foils and vacuum dried at 50 °C for 12 h to obtain the plain cathodes. The CB-S electrode was also prepared by mixing sulfur, Super P and PVDF with a mass ratio of 6:3:1. The sulfur loading in the electrodes was basically fixed at 1.1 mg cm -2 without specific illustration.

Preparation of 3D-printed Co/Co-N@NPCF-S cathode
Firstly, 50 mg of graphene oxide (GO) powder was sonicated in 2 mL deionized water for 2 h to obtain GO solution. Afterwards, 450 mg of Co/Co-N@NPCF-S and several drops of water were added into the above solution and stirred for 12 h to form the uniform ink. The as-prepared ink was then transferred into a syringe tube attached with a blunt-tip needle (inner diameters of needle tips: 0.46 mm, 26 Ga) for direct ink writing 3D printing. And the 3D printing process was conducted by a modified 3D printing system (RZC-30WK, Dongguan origin Automation Technology Co. Ltd).
Under the appropriate printing speed and air pressure, the ink was printed as electrodes of tunable thicknesses with an area of 1.0 cm -2 . Finally, the 3D-printed architectures were freeze-dried to obtain the 3D-printed Co/Co-N@NPCF-S cathodes with various sulfur loadings.

Characterizations
Morphology of the samples was observed with the field-emission scanning electron microscope (FESEM, JSM 7500F) and field-emission transmission electron microscope (FETEM, Talos F200S). X-ray diffraction (XRD) patterns were obtained on an X'Pert PRO X-ray diffractometer at a current of 40 mA and voltage of 40 kV with Cu K  radiation. X-ray photoelectron spectroscopy (XPS) was examined by the Thermo Scientific ESCALAB 250Xi equipped with an Al K  X-ray source at an energy of 1486.6 eV. Brunauer-Emmett-Teller (BET) nitrogen adsorption/desorption isotherms were measured by using a Quantachrome Autosorb-iQ/MP ® XR system. Thermogravimetric analysis (TGA) was carried out on a NETZSCH TG209F1 Libra device.
For the visualized adsorption test, Li 2 S 6 as a representative of LiPSs, was prepared by mixing lithium sulfides (Li 2 S) and sublimed sulfur at a molar ratio of 1:5 in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME) (1:1, v/v) followed by vigorous stirring for 24 h. A 5 mM Li 2 S 6 solution was used for the adsorption test. Typically, Co/Co-N@NPCF and NPCF were added to 3 mL of Li 2 S 6 solutions respectively, with the pure Li 2 S 6 solution as a comparison. Then, digital photos of the macroscopic adsorption toward LiPSs by different materials were taken after resting for 12 h.

Electrochemical measurements
The coin cells were assembled in an argon-filled glove box with lithium foil as the counter electrode, 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DOL/DME (1:1, v/v) containing 2% LiNO 3 as the electrolyte, a Celgard 2500 membrane as separator, and a plain or 3D-printed cathode as the working electrode.
The galvanostatic discharge/charge tests were performed between 1.7 -2.8 V (vs. Li/Li + ) by a LAND 2001A battery testing system. Cyclic voltammetry (CV) curves were carried out on an ARBIN BT2000 system in the voltage range of 1.7 -2.8 V at different scan rates. Electrochemical impedance spectra (EIS) measurements were carried out by an electrochemical workstation in the frequency range from 10 5 Hz to 1 Hz under the automatic sweep mode.
The electrodes used for symmetrical batteries were prepared without elemental sulfur. The host material of Co/Co-N@NPCF or NPCF was dispersed in NMP with PVDF binder at a mass ratio of 9:1 to form a uniform slurry. The slurry was coated on the aluminum foils and dried at 80 °C under vacuum for 12 h to prepare the electrode disks as both working and counter electrodes. In addition, a Celgard 2500 membrane was used as the separator and 40 μL of 0.5 M Li 2 S 6 in DOL/DME (1:1, v/v) solution with 1.0 M LiTFSI was added as electrolyte. CV measurements of the symmetrical batteries were performed at a scan rate of 50 mV s -1 between -0.8 V and 0.8 V.
The electrodes used for Li 2 S nucleation measurements were similarly prepared as above. A Li 2 S 8 solution was prepared by mixing Li 2 S and sulfur at a mass ratio of 1:7 in tetraglyme solution with 1 M LiTFSI. Co/Co-N@NPCF or NPCF cathodes was used as the working electrode and the bare lithium foil works as the counter electrode.                           Table S1. Simulation results of the kinetic parameters of Co/Co-N@NPCF-S, NPCF-S and CB-S electrodes.